The air filter is an extremely important part of a car's ignition system. When an engine burns, almost all of the fuel is turned to gas. This gas powers the pistons down, then is ejected from the engine so a new compression cycle can begin. There are many things in the air, however, that don't burn cleanly or at all. If tiny pieces of dirt, road salt, pollen or other contaminants get into your car's air intake, they can damage pistons, clog valves and generally shorten the life of your car. The air filter stops these contaminants from getting in in the first place.
Air filters on most cars are rectangular, pleated screens called paper filters. Although the name may make some people think these filters are flimsy, they are actually made from a sort of felt which is much tougher than normal paper. They are porous, with tiny holes big enough to let air through but small enough to catch other particles on the way in. Air first flows through the filter, then into the intake manifold. It then mixes with gas and enters the pistons for combustion.
Paper air filters are cheap, do not restrict the air flow very much, and effectively remove most or all of the dirt coming in. This makes them the best choice for most automotive uses. For off-roading or other areas where the filter needs to absorb a lot of dirt, other materials are sometimes used. Foam filters can absorb a lot of dirt very efficiently, and do not restrict the flow of air much, making them a popular choice. Another popular off-road filter is the oil bath filter. Air is sent up and down in a maze between a filter mesh and a tub of oil. Large particles get stuck in the oil, whereas smaller bits of dirt get trapped by the mesh.
